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In sinus rhythm left atrial pressure falls from its "a" and "v" peaks in "x" and "y" descents. The "x" descent is usually interrupted by a small positive transient, the "c" wave. In atrial fibrillation "a" waves and "x" descents disappear and systolic "c-v" waves remain (Fig. 1). There appears little doubt that "a" waves are caused by atrial contraction, and "v" waves by accumulation of blood in the atrium during ventricular systole. "c" may fairly be attributed to early systolic bulging of the mitral leaflets, and "y" to diastolic atrial decompression. The cause of "x" descent is not certain. Early systolic movement of the atrioventricular ring towards the apex of the heart has been held responsible by Evans (1941), Bloomfield et al. (1946),

Symposium on diagnostic methods in the study of left-to-right shunts. Circulation. 16 (791). 1957. KETY, S. S. Quantitative détermination of cérébral blood flow in man. Methods in Médical Research, vol. 1 (204i. The Year Book Publishers Inc.. Chicago. 1948. MORROW. A. G., R. J. SANDERS and E. BRAUNWALD. The nitrous oxide test : An improved method for the détection of left-to-right shunts. Circulation, 17 (284). 1958. SANDERS. R. J. and A. G. MORROW. The diagnosis of circulatory shunts by the nitrous oxide test. Improvements in technic and methods for quantification of the shunt. Circulation, 18 (856i. 1958.